The invention relates to a device for detecting a pulsating variable in an internal combustion engine by sampling a measurement value in synchronization with a crankshaft angle every half-period of the pulsating variable.
It is known that significant variables in internal combustion engines such as, for example, the vacuum in the intake pipe are quasi-continuous variables which pulsate or periodically fluctuate with the power cycle of the internal combustion engine. However, it is the mean value of these variables which is needed for regulating the internal combustion engine.
Devices for detecting a periodically fluctuating variable, for example the vacuum in the intake pipe, and for determining the mean value of this variable are already known. Thus, in German Offenlegungsschrift 3,223,328, the pressure is measured ignition-synchronously once per ignition interval and from the instantaneous value of the pressure thus obtained the mean value is calculated. In case of a phase shift of the pressure signal which is caused, for example, by fuel condensates which can become deposited in the hose between the intake pipe and pressure transducer, however, the solution proposed in German Offenlegungsschrift 3,223,328 leads to errors in the averaging.
In German Offenlegungsschrift 3,212,561, the intake pipe pressure is determined by fixed-angle dual sampling per ignition interval. If only the fundamental oscillation is present, the dual sampling of the intake pipe pressure per ignition interval would be sufficient for determining the exact mean value of the intake pipe pressure. At certain rotational speeds, however, higher-frequency components are superimposed on the periodic intake pipe pressure variation which could lead to errors occurring in the averaging even with dual sampling.